1. Field of the Invention
The present invention is generally related to a brushless direct current (DC) fan and in particular to a brushless DC fan of which the driving circuit comprises a square wave generation circuit for generating a square wave signal indicating the operation of the fan so as to allow ready monitoring of the fan, wherein the driving circuit comprises two transistors alternately conducted on by a Hall device to alternately energize two windings for driving the fan blades to rotate and the square wave generation circuit is coupled to the collector of one of the transistor in order to provide the square wave signal by processing an electrical signal obtained from the collector of the transistor.
2. Description of the Related Art
Direct current (DC) fans have a variety of applications, such as heat dissipation for personal computer. Generally speaking, the DC fan comprises a plurality of permanent magnetic north poles and south poles alternating each other along a circular inner circumference of the fan with a plurality of fan blades 10 fixed to the outer side of the magnetic poles to define a rotor of the fan. A stator of the fan is disposed inside the space surrounded by the magnetic poles, comprising two sets of electromagnets 11 and 12 which are respectively controlled by two sets of windings 13 and 14. One of the electromagnets, for example the electromagnet 11, has a Hall device 15 attached thereon to serve as a sensor and controller. The Hall device 15 controls the rotation of the fan blades 10 via a driving circuit. The DC fan is shown in FIG. 1.
The fan driving circuit, as shown in FIG. 2, comprises two transistors 16 and 17 of which the collectors are respectively connected to the windings 13 and 14. The base of the transistor 16 is connected to an output terminal of the Hall device 15 and the collector of the transistor 16 is also connected to the base of the transistor 17. When the rotor rotates, the Hall device 15 detects for example one of the north poles of the permanent magnet and energizes for example the winding 13 via the transistor 16 which causes the electromagnet 11 to generates the same polarity as the permanent magnet and thus an repulsive force therebetween to drive the rotor for a predetermined angular displacement. With the alternate arrangement of the permanent magnet north and south poles, the angular displacement causes an opposite polarity to be detected by the Hall device 15 which energizes the winding 14 via the transistor 17. This causes the rotor to be further driven and by repeatedly detecting the polarities and driving the rotor via the windings 13 and 14 in an alternate manner, the fan rotates continuously.
In certain cases, such as in a computer application, the DC fan is disposed in a closed space (inside the computer casing) and it is in general difficult, if not impossible, for the computer users to monitor or watch the operation of the fan. Once the fan malfunctions, the heat dissipation of the computer is affected and this may cause significant problem in the operation of the computer central processing unit (CPU). Due to such a situation, monitoring the operation of the DC fan may be needed for certain applications.
One way to monitor the operation of the DC fan is to detect an electrical signal that is generated by the transistors 16 and 17 to energize the windings 13 and 14. A waveform of the electrical signal obtained from the collector of either one of the two transistors 16 and 17 is shown in FIG. 3. Such a waveform is comprised of very irregularly shaped pulses and is an imperfect signal to be used for monitoring the operation of the fan for it is difficult to handle as it is irregular.
To solve such a problem, Taiwan Patent Publication No. 261303 (Application No. 83214410) discloses a brushless DC fan structure which is shown in FIG. 4 for reference. The Taiwanese patent teaches using an external circuit to process the irregular signal. The external circuit is connected to a terminal L1 of the winding of the DC fan, which is designated FAN in FIG. 4 (such as the collector of the transistor 17 in FIG. 1) and comprises a first transistor T1 which is connected to the terminal L1 via a capacitor C1 and a resistor R1. The emitter of the first transistor T1 is grounded via a capacitor C2 and is also connected to the base of a second transistor T2 via a resistor R2 so as to obtain square wave signals from the collectors and emitters of the two transistors T1 and T2. The signals so obtained are applied to an external monitoring circuit to monitor the operation of the fan.
Although such a prior art fan structure allows a user to obtain a desired square wave signal from the fan for monitoring purpose, the signal generation circuit thereof is comprised of a number of additional elements. This is very disadvantageous in view of the limited space that is available inside the DC fan. Thus, the circuit has to be externally mounted to the fan. This increases the overall size of the fan and is contrary to the trend of minimization of the DC fans.
To this point, it is aware that obtaining a square wave signal from the fan in order to monitor the operation of the fan or to feedback to the fan to control the speed thereof is very helpful in controlling the operation of the DC fan and is also a trend in the design of the DC fan. However, currently, the ways of generating the square wave signal from the operation of the fan are not good enough in respect of the size and the practicability.
It is thus desirable to provide an improvement of the brushless DC fan of which a square wave generation circuit that may be built inside the DC fan to provide a desired square waveform to be processed for monitoring and control purpose.